An installation for transmission of high-voltage direct current between two alternating-voltage networks comprises two converter stations, each one connected on its ac side to a separate one of the alternating-voltage networks, and a common dc connection. The dc connection may be in the form of an overhead line and/or a cable and also in certain parts consist of ground or water instead of a metallic conductor. In certain cases the converters are erected in the immediate vicinity of each other, so-called back-to-back erection, whereby the dc connection may consist of short busbars. Each one of the converter stations comprises a converter, usually at least one converter transformer for connection of the converter to the alternating-voltage network, as well as shunt filters for generation of reactive power and filtering of harmonics. The converters are normally line-commutated, current-source converters, by which is to be understood that the current commutation between the valves of the converters takes place by means of voltages occurring in the alternating-voltage network, and that the dc connection, viewed from the converters, occurs as a stiff current source. For the purpose of reducing the harmonics generated by the converters, especially the 5th and 7th harmonics, each one of the converters usually consists of two mutually series-connected six-pulse bridges, each one connected to the alternating-voltage network via a separate secondary winding on the converter station, the transformer being connected such that the secondary windings have a mutual phase shift of 30.degree..
During normal operation, one of the converters, hereinafter referred to as the rectifier, operates in rectifier operation, and the other, hereinafter referred to as the inverter, operates in inverter operation. Control equipment for the respective converter generates a control signal corresponding to a control angle .alpha. at which firing pulses are applied to the valves of the converters. For the purpose of minimizing the consumption of reactive power by the converters, and reducing the stresses on components included in the converter stations, it is advantageous to control the rectifier with the smallest possible control angle .alpha. and to control the inverter with a control angle which results in the smallest possible extinction angle .gamma. (margin of commutation) without jeopardizing the controlled operation. The control system of the installation is, therefore, usually designed such that the inverter is controlled to a suitable maximum direct voltage for the operating conditions of the installation, taking into consideration safety margins with respect to commutating errors, voltage variations on the ac network, and other deviations from nominal operation which may occur. The rectifier is controlled in current control, the reference value of which is formed in dependence on a current order, which in turn is formed in dependence on a power order and the prevailing direct voltage in such a way that the direct current and hence the transferred active power remain at a desired value.
To ensure that the valve, at the moment of firing, has sufficient firing voltage, that is, forward voltage in blocked state, the control equipment of the rectifier further comprises a lower limitation of the control angle such that its minimum value is not lower than a preselected lowest value. This limitation is conventionally achieved by sensing the voltage across the valve with a measuring device, whereby firing pulse for the valve is generated only when the sensed voltage exceeds a preselected value.
Usually, the control equipment for rectifiers and inverters is designed identically, whereby in the rectifier a current controller is activated and in the inverter control equipment for a control with the aim of maintaining the extinction angle at, but not lower than, a preselected lowest value is activated.
For a general description of the technique for transmission of high-voltage direct current, reference is made to Erich Uhlmann: Power Transmission by Direct Current, Springer Verlag, Berlin Heidelberg New York 1975, in particular pages 125-136.
Between the control angle .alpha., the extinction angle .gamma. and the overlap angle u, the known relationship .alpha.+u+.gamma.=180.degree. prevails. It is thus desirable to determine the control angle for the inverter such that the extinction angle (margin of commutation) remains at a preselected lowest value.
U.S. Pat. No. 4,210,956 describes a method for control of an installation for transmission of high-voltage direct current. According to the method, for each one of the converters included in the installation, the control angle of the converter is calculated based on given values of voltage and current at each converter in the installation. The calculation is performed by means of known relationships between current, voltage, commutating reactance and control angle with a view to achieving a situation whereby the installation in its entirety can be operated under stable conditions. The minimum permissible control angle, that is, the minimum firing voltage, and the minimum permissible extinction angle, are thus regarded as limit values in these calculations. The converters are not series-compensated and the method appears essentially to be intended for so-called multiterminal systems, in which more than two converter stations are connected to a common dc connection.
U.S. Pat. No. 4,264,951 describes equipment for control of an installation for transmission of high-voltage direct current. The equipment comprises, in addition to control means for control on constant current, constant voltage and constant extinction angle, also a device which, based on applied values of an alternating voltage and an alternating current sensed at the converter, calculates the limit values for the control angle of the converter at which limit values the losses in the damping circuits of the converter valves amount to a certain value. Output signals from the mentioned devices are supplied to a selector means in which a control-angle signal is selected from any of the mentioned control means while taking into consideration that the calculated limit values are not exceeded.
U.S. Pat. No. 4,563,732, especially FIGS. 1 and 2 with the associated description, shows a method and a device for control of a converter in inverter operation. A current controller comprises a proportional-amplifying and an integrating member. The proportional-amplifying member is supplied with a reference value for the current in the dc connection subtracted by a sensed value of this current. The integrating member is also supplied with the reference value of the current in the dc connection subtracted by the sensed value of this current but, in addition, subtracted by a current margin. The sum of the output signals from the proportional-amplifying member and the integrating member constitutes the control angle order delivered by the control system to the control pulse device of the inverter. Under stationary conditions, the output signal from the proportional-amplifying member is equal to or near zero whereas the input signal to the integrating member consists of the current margin. The integrating member comprises a controllable limiting input by means of which its output signal may be limited and the value of the control angle .alpha. ordered by the inverter is thus determined by the mentioned limiting signal. The limiting input is supplied with a limiting signal, formed by a calculating circuit and corresponding to a control signal for the inverter and calculated based on relationships, known to the main circuit of the converter station, between the ideal direct voltage of the converter at the control angle equal to zero, a direct current corresponding to the reference value of the current controller, the impedance of the alternating-voltage network, a preselected extinction angle (margin of commutation) and the corresponding control angle. The above-mentioned limiting signal may be corrected by a slowly acting control circuit with negative feedback of a sensed value of the extinction angle which is compared with a reference value for this angle. Control pulses are generated, in addition to the above-described control system, also by an extinction angle control circuit, based on the fact that a certain minimum voltage-time area is required for the semiconductor elements included in the valves, usually thyristors, to be able to resume their blocking state after decommutation. On the basis of a sensed value of the commutating voltage and while assuming a certain shape of the curve therefor, the remaining voltage time area is continuously calculated up to the zero crossing of the commutating voltage, corrected for the length of the commutating interval which is dependent on the direct current. When the corrected voltage time area is equal to the smallest voltage time area required, generation of the firing pulse of the valve is initiated. The firing pulse, which is dependent on the current controller and on the extinction-angle control circuit, is supplied to an OR circuit such that that of the two pulses which first arrives at the OR circuit initiates firing of the valve.
It has proved that the control system of the inverter, designed in conventional manner, is sensitive to disturbances in stationary operation. Since the control of the inverter per se results in a negative current/voltage characteristic occurring in the inverter, a small voltage reduction on the alternating-voltage network of the inverter may lead to an avalanche-like growth of the direct current. To obtain a stable control of the current, the rectifier must, through its current control, exhibit a positive current/voltage characteristic to compensate for the negative characteristic in the control system of the inverter. A high capacitance to ground in the dc connection, which occurs when the dc connection is in the form of a long cable, means that the current control of the rectifier and the control system of the inverter are to a certain extent disconnected from each other. Fast voltage reductions in the alternating-voltage network, for example in case of short-circuit faults or single-phase or multi-phase ground faults, may lead to the voltage at the inverter breaking down. In order for the inverter, through its control-angle control, to be able to counteract such events, in conventional, non-series-compensated converter stations the control system must, therefore, during stationary operation, operate with a commutating margin greater than that which corresponds to normal safety with respect to commutating errors. However, this entails, on the other hand, increased reactive power consumption during stationary operation with ensuing higher costs for compensation and an uneconomical dimensioning of the installation in its entirety.
It is known to series-compensate converter stations by connecting converter bridges, comprised in the converter station, to the respective alternating-voltage network via series capacitors. This results in several advantages. The series capacitors are charged periodically by the current traversing it and the voltage thus generated across the capacitors provides an addition to the commutating voltage across the valves of the converter. The commutating voltage becomes phase-shifted in relation to the voltages of the alternating-voltage network in such a way that, with control and extinction angles still related to the phase position for the voltages of the alternating-voltage network, the valves in rectifier operation may be controlled with control angles smaller than zero and, in inverter operation, with extinction angles smaller than zero (although the commutating margin related to the commutating voltage of the valve is, of course, greater than zero). This makes possible a reduction of the reactive power consumption of the converters. This reduces the need of generation of reactive power in the shunt filters and these may thus be dimensioned substantially based on the need of harmonic filtering. The charging current of the capacitors and hence their voltage are proportional to the direct current in the dc connection and by suitable dimensioning of the capacitors, the dependence of the overlap angle on the magnitude of the direct current may be compensated. This means that the series compensation contributes to maintain the commutating margin of the valves also in case of fast current transients. Also the dependence of the commutating margin on the amplitude of the alternating-voltage network is influenced in a favourable direction by the series compensation in that the above-mentioned negative current/voltage characteristic in the converter control is influenced in a stabilizing direction and, by suitable choice of series capacitors, can also be caused to be positive.
A general description of the mode of operation of the converter station with series capacitors introduced into the ac connections between the converter transformer and a converter in a six-pulse bridge connection is given in John Reeve, John A. Baron, and G. A. Hanley: A Technical Assessment of Artificial Commutation of HVDC Converters with Series Compensation (IEEE Trans. on Power Apparatus and Systems, Vol. PAS-87, Oct. 1968, pages 1830-1840).
Thus, it is desirable in many contexts to series-compensate converter stations of the kind described above.
However, series compensation of the converter station means that the commutating voltage of the valves is dependent on both amplitude and phase for the current-dependent voltage across the respective series capacitor. During series compensation, thus, the commutating voltage of the valves cannot be directly derived from voltages sensed in the alternating-voltage network in the way which is possible in non-series-compensated converter stations, and on which the above-mentioned principles of control of the extinction angle and the firing-voltage conditions of the rectifier are based.
For the main circuits of the series-compensated converter stations, current/voltage equations may be set up in a known manner, with the control angle .alpha. (related to the voltages of the alternating-voltage network), the direct current Id, the ideal no-load direct voltage Udi0 and the commutating margin .gamma..sub.m (in inverter operation) as variables. If in these equations a constant preselected value .gamma..sub.p of the commutating margin of the valve is assumed, the control angle .alpha. may be calculated with the direct current and the ideal no-load direct voltage as variables.
However, in series-compensated converter stations, the current/voltage equations become considerably more complicated than in non-series-compensated ones and the calculation cannot be performed quite simply, as in U.S. Pat. No. 4,563,732, such that the control angle is explicitly solved from the equation. A calculation of the control angle based on these equations must be suitably carried out by iteration, which places heavy demands on calculation speed and/or the supply of the calculation capacity.
The control of an inverter, wherein a calculating circuit is adapted to explicitly form, starting from the current/voltage equations of the main circuits, a signal corresponding to the desired control angle, as described in the above-mentioned U.S. Pat. No. 4,563,732, therefore becomes disadvantageous in connection with a series-compensated converter.
A direct measurement of the extinction angle is rendered considerably more complicated by the introduction of series compensation in that an individual measurement would be required at each valve, thus a considerable complication and cost.